The present invention relates to an improved photoconductor used in electrophotographic reproduction devices, having a charge generating layer and a charge transport layer, which exhibits reduced room light and cycling fatigue and potentially improved sensitivity of the photoconductor.
Azines, which are the products of condensing remaining NH2 of a hydrazone with a carbonyl compound, have been reported for electrophotographic applications, both as a transport molecule and a dopant in hydrazone-containing charge transport layers (U.S. Pat. No. 6,004,708, commonly assigned with this application). T. Ueda et al. from Minolta Camera Company (DE 3716982, JP 62006262, JP 61209456) claims several series of hydrazones and azines as charge transporting materials. Some other examples can be found in JP 61043752, JP 61043753 and JP 61043754.
The azine compounds used in this invention which are derivatives of fluorene, when used as charge transport molecules do not exhibit good injection into charge generation molecule due to their high oxidation potentials. The highlight of this invention is that these derivatives can be used as an efficient light filter and/or excited state quencher for charge transport formulations, which results in significant reduction in room-light fatigue and cycling fatigue. As a result, print defects caused by positive or negative fatigue are significantly reduced.
A good photoconductor should have stable electrostatic characteristics and robust mechanical properties under severe conditions, e.g. light exposure and touch by human hands during handling and storing. However, fatigue induced by light exposure is still quite common in present-day products. This fatigue can be so severe that some charge transport formulations containing hydrazone compounds results in the increase of photodischarge by several hundred volts if no light absorber is used. It has been found that photoisomerization and photochemical reactions are mostly responsible for fatigue. For example, p-(diethylamino)benzaldehyde diphenylhydrazone (DEH) undergoes a photochemically induced unimolecular rearrangement to an indazole derivative, 1-phenyl-3-(4(diethylamino)- 1-phenyl)-1,2-indazole. Excellent review of mechanism of photoinduced fatigue can be found from the following articles: J. Pacansky, et al.; Chem. Mater. 1992, 4 401; T. Nakazawa, et al.; Chem. Lett. 1992, 1125; E. Matsuda, et al.; Chem. Lett. 1992, 1129. Thus, it is important to find an effective light absorber for a charge transport material to filter away the harmful wavelength light. An example can be found in H. W. Anderson and B. Champ, U.S. Pat. No. 4,362,798, in which Acetosol Yellow was added to formulation of p-(diethylamino) benzaldehyde diphenylhydrazone (DEH). Most recently, the foregoing commonly assigned U.S. Pat. No. 6,004,708 was issued to M. Bellino, B. Champ and W. Luo, which claims using azine derivatives to reduce fatigue of DEH charge transport properties.
Fluorenyl azines described in this disclosure are known compounds (structure see Scheme 1). For example, 9-[p-(diethylamino)benzylidenehydrazono]fluorene has been disclosed in JP 57138644 and JP 59195659 as a charge transport material, however, no work has been done to use these azine derivatives with triarylamine molecules, e.g. N, Nxe2x80x2-diphenyl-N,Nxe2x80x2-di(m-tolyl)-p-benzidine (TPD), to mitigate light fatigue.
This disclosure pertains to significant improvement of electrostatic stability of a photoconductor by formulating an azine derivative and an antioxidant into charge transport formulations containing at least one triarylamine or a combination of a tariarylamine and a hydrazone. Very low percentage of azine, even as low as 0.5%, is efficient to stabilize both sensitivity and dark decay. Particularly when a combination of azine and antioxidant is used, additional advantage of reduced dark decay and improved crazing resistance has been seen.
The structures of the azine and charge transport compounds used in this invention are shown in Scheme 1 and 2. 
Where R1 and R2=ethyl, phenyl; R3=hydrogen, phenyl. 
Addition of azine compounds at about 0.5-about 5% by weight in a charge transport layer having triarylamine, binder and other materials making up the remaining 95-99.5 weight stabilizes electrostatic characteristics of a photoconductor upon exposure to light. As a result, print defects from light fatigue are reduced or eliminated. Moreover, a combination of antioxidant and azine improves both initial electrical properties and fatigue induced by electrical cycling or room light exposure.